This invention relates to improvements in heterojunction bipolar transistors (xe2x80x9cHBTxe2x80x9d) and, more particularly, to a practical HBT construction that obtains both an improved figure of merit in HBT performance and an improved maximum frequency of oscillation for the HBT.
Heterojunction bipolar transistors (xe2x80x9cHBTxe2x80x9d), which find application, as example, in high speed (eg. high frequency) digital switching, are known and well defined in the technical literature. The HBT is a layered structure, such as illustrated in side view (width) in FIG. 1. That layered structure includes a semiconductor substrate 1, a subcollector 3, collector 5, base 7 and emitter 9 stacked one atop the other in an integral assembly. Metal contacts are formed on the emitter, base and subcollector for appropriate connection to external power supply and/or other electronic circuits. Those metal contacts include a subcollector metal 11, a base metal 13, and emitter metal 15.
In top view the familiar HBT appears as in FIG. 2, wherein the subcollector 3 defines a large rectangle in shape. Within that rectangular region and, as viewed from the top, the rectangular shaped emitter metal 15 is formed atop and almost covers the rectangular region of the emitter layer 9, also rectangular in shape. The base metal 13 is formed upon the base layer 7 and essentially forms a picture-frame like structure surrounding emitter layer 9. The outer dimensions of base metal 13 fall short of the rectangular area defined by the base layer 7, the length, l, and width, w, of which, is referred to as the base mesa length and base mesa width. Collector 5 directly underlies base 7 and is not visible in this view. The collector metal 11 that is formed on subcollector layer 3 is an elongate rectangle in shape. It is noted that substrate 1 is omitted in this view.
A common HBT is the Indium Aluminum Arsenide, Indium Gallium Arsenide and Indium Phosphide (xe2x80x9cInAlAs/InGaAs/InPxe2x80x9d) HBT, and the invention is more easily understood with that type of HBT as the example. Such HBT transistors are grown on an epi-layer of InP material, a crystalline wafer that is sliced from a large crystal (the boule) that is grown from a xe2x80x9cseed.xe2x80x9d Typically the wafer is supplied by a specialty manufacturer, the crystal grower, to the transistor manufacturer and that wafer contains straight edges along two sides of the wafer. Those two straight edges are perpendicular to one another. Each straight edge is aligned with (formed parallel to) a respective plane of the crystal of the wafer as an aid to mask alignment for the photo-lithographic procedures used during manufacture of the HBT. Using conventional crystal growing techniques (such as molecular beam epitaxy, MBE) the additional layers necessary to form the transistor are grown on the face of the crystalline InP wafer, effectively further growing the crystal structure of the wafer in height.
The first additional layer, subcollector 3, is formed of a layer of InGaAs over a layer of InP material that has been doped negative, that is, the InGaAs and InP contains an impurity, typically Silicon, which gives the semiconductor layer a low electrical resistivity. The low resistivity enables the layer to serve as an electrical conductor to provide an electrical path from the collector metal 11 to the underside of collector 5. The next layer, the collector of the HBT, is grown using InP material, which is doped negative, but at a lower concentration than the doping of the subcollector. The collector is followed by growing a layer of InGaAs, the base 7 of the HBT, which is doped heavily positive using an impurity, customarily of Beryllium. The final semiconductor layer, the emitter 9, is grown of InAlAs (Indium Aluminum Arsenide) that is medium doped negative with Silicon.
Once the laminate-like crystal structure is completed, the masking and etching procedures follow to define the shape and size of the various layers, such as was illustrated in FIG. 2. The InAlAs/InGaAs/InP wafer structure is first masked and then etched with a phosphoric acid based solution that etches the top InAlAs layer. Then the structure is masked again and the InGaAs base layer is etched. Then the structure is masked and then etched with a hydrochloric (xe2x80x9cHClxe2x80x9d) acid based solution that etches the InP. For greater detail of the known processes, the reader is referred to the technical literature.
The available space (xe2x80x9creal estatexe2x80x9d) on the wafer is sufficient to accommodate perhaps thousands of HBT""s. Thus the mask contains the individual masks of identical layer geometry arranged in rows and columns to permit simultaneous fabrication of large numbers of HBT""s and circuits containing HBTs. Those HBT""s may later be cleaved (xe2x80x9cdicedxe2x80x9d) from the wafer and separated for individual packaging or retained on the substrate for use in a semiconductor array. For an understanding of the present invention only an individual HBT structure needs to be considered.
Once etching is completed, metal contacts are deposited in place on the subcollector, base and emitter layers. Typically a layer of a dielectric or polyimide, a plastic insulator, is used to cover the semiconductor, except for the regions on which the metal contacts are to be deposited. After deposition of the contact metal the conductive leads to the metal are deposited on and extend over that dielectric or polyimide insulator layer.
In the InAlAs/InGaAs/InP HBT of FIG. 1, as example, substrate 1 is about 500-600 microns in thickness, the subcollector 3 about 4,000 Angstroms ({fraction (1/10,000)}th micron) thick, the collector 5 about 4,000 Angstroms thick, base 7 about 400 Angstroms and emitter 9 about 2,500 Angstroms in thickness. The metal contacts are about 2000 Angstroms in thickness. The foregoing dimensions illustrate the relative thickness (or scale) of the various regions of the HBT.
The function of the elements of the HBT and its theory of operation are well documented in the technical literature, and is not here repeated. Basically with the base, emitter and collector properly electrically biased, the HBT serves as an electronic switch or amplifier. As example, that electronic switch conducts current or not between emitter and collector in dependence on the application of a voltage of appropriate level to the base.
Being used in high speed digital application, the higher the operating frequency at which the device operates and the higher the figure of merit of a given HBT design, the better. The design consideration for each of those two factors are different and contradictory. It is found that enhancing the one factor is disparaging of the other factor and vice-versa.
As study has shown, the most important figures of merit of high frequency performance heterojunction bipolar transistors (HBT) are fmax and fxcfx84. where fmax is the frequency at which unilateral gain becomes unity and fxcfx84 is the current-gain cutoff frequency. An approximate expression of fmax is (1)             f      max        =                            f          τ                          8          ⁢          π          ⁢                      xe2x80x83                    ⁢                      R            B                    ⁢                      C            BC                                ,
where RB is the parasitic base resistance and CBC is the base collector capacitance. Equation (1) shows the relation of fmax to fxcfx84, RB and CBC. To enhance fmax, fxcfx84 should be increased and RB and CBC should be minimized.
RB is the parasitic base resistance, which is essentially a combination of the ohmic contact resistance and the base access resistance of the transistor. FIG. 3 is a simplified diagram that shows those two resistive components. The base contact resistance is dependent upon the base layer material, the base layer doping and the ohmic metal technology used to fabricate the base contact, which are nearly independent of the base layer thickness. On the other hand, the base access resistance is inversely proportional to base layer thickness. For a given material and doping, thick base layers provide low base access resistance, while thin base layers possess high access resistance. Consequently, to reduce RB, the base layer thickness should be increased.
CBC is the base to collector capacitance. That capacitance is a function of the thickness of the collector layer, the dielectric constant, xcex5xcfx84 of the collector material, and the base-collector junction area. A lower CBC is obtained with thicker collector layers, smaller junction areas and lower dielectric constants. The base-collector area is given by the base mesa width, w, and length, l, as illustrated in FIG. 2.
Returning to FIG. 1, the current gain cutoff frequency, fxcfx84, is the frequency at which the transistor small signal current gain is unity. That cutoff frequency can be estimated using the following equation:             f      ⁢              xe2x80x83            ⁢      τ        =          1              2        ⁢                  xe2x80x83                ⁢                  πτ          EC                      ,
where xcfx84EC is the emitter to collector electron transit time. In other words, xcfx84EC is the time that the electrons require to travel from the emitter 9 to the subcollector 3, and that transit time depends principally on the thickness of the base 7 and collector 5 layers. For thick layers, the transit time is greater; and for thin layers the transit time is shorter. This flow is depicted in FIG. 1 as the vertical wide dash-line arrow extending from the emitter 9 to the subcollector 3. Thus, to increase fxcfx84, the thickness of the base and collector layers should be reduced.
From the foregoing dependencies, the compromise that is made in the design of an HBT to obtain an optimum fmax and fxcfx84 becomes evident. For high fxcfx84, the HBT structure must possess thin base and collector layers, but that degrades fmax by increasing RB and CBC.
A prior technique for fabricating HBT""s with reduced base to collector capacitance is known as the xe2x80x9ctransferred substratexe2x80x9d technique. In that technique the emitter and base of the HBT are fabricated on the front side of the InP wafer and the wafer is then mounted front side down on a carrier or surrogate substrate. The original semiconductor substrate is then removed and the collector structure and the remaining elements of the circuit are fabricated upon the surrogate (transferred) substrate. That technique is described in U.S. Pat. No. 5,318,916 and in an article xe2x80x9c48 GHz Digital ICs Using Transferred Substrate HBT""s,xe2x80x9d M. Rodwell et al, GaAs IC Symposium Technical Digest, November 1998.
The Rodwell et al. Technique possesses an inherent disadvantage. In addition to the complexity of the fabrication process, there is a discrepancy between the coefficient of thermal expansion of the semiconductor layer in which the HBT devices are fabricated and the thermal expansion coefficient of the surrogate substrate. That difference creates alignment problems once the circuits are on the surrogate substrate and limits the size of the circuit that may be fabricated.
In a typical HBT, much of the base-collector capacitance originates in the areas under the base ohmic contact. These areas, illustrated between the dotted vertical lines 6 and 8 and the respective adjacent side edges of collector 5 at the right and left hand sides, shown in FIG. 1, do not participate in significant vertical current conduction in collector 5 and only contribute to an increase in base-collector capacitance, CBC. Hence, those areas may be regarded as excess. By removing those excess regions of the collector, a cantilevered base with undercut collector structure is created, and the CBC is reduced substantially by the removal of a relatively high dielectric material, approximately 13 in InP. The present invention is of that approach.
Such an approach to improving the HBT structure through undercutting the collector to reduce base-collector capacitance is also the subject of an earlier publication by Miyamoto, Rios, Dentai and Chandrasekhar, in an article xe2x80x9cReduction of Base-Collector Capacitance by Undercutting the Collector and Subcollector in GaInAs/InP DHBT""s,xe2x80x9d The Electron Devices Letters, Vol. 17, No. 3, March 1996, pages 97-99.
In the HBT device that Miyamoto et al thought practical, the collector layer was undercut, apparently about the periphery thereof, removing considerable high dielectric material of the collector material. Lacking robustness in the resultant structure, the removed material was replaced with and covered overall with an insulating material of a lower dielectric constant, polyimide, the dielectric constant of which is about 4. Applied as a fluid (and later cured) the polyimide is poured over the semiconductor die and seeps into and fills all the crevices, and provides a covering sheet for the semiconductor. To complete the Rodwell et al HBT device, extra processing is used to attach leads to the electrical contacts. Namely, Oxygen plasma etching is performed to open windows (passages) in the polyimide covering to permit access to the collector metal and the emitter metal. Then pad metal was evaporated on the hardened polyimide to fill those windows and provide a through-hole conductor for the electrical connection to the contact metal on the semiconductor layers.
As a disadvantage, the dielectric constant of polyimide is significantly greater than air or other conventional gases that might be hermetically sealed in the package of the HBT (or HBT""s) or in which environment the HBT may be operated. Hence, the HBT design proposed by Miyamoto et al would not possess as low a base-collector capacitance, CBC than otherwise appears possible.
As an advantage, the present invention does not require planarizing the wafer (and HBT transistors in the wafer) with a deep coating of polyimide material. Hence, the present avoids any necessity for etching openings through a polyimide fill in order to attach electrical leads to the elements of the HBT. As another advantage, the present invention obtains inherently a lower CBC than the HBT of Miyamoto et al and, hence, a higher operating frequency.
As a further advantage, apart from changes in masking definition, the invention requires only existing technology for manufacture of the HBT, employing the same materials and processing techniques as in the prior art HBT""s.
Accordingly, a principal object of the invention is to provide a HBT that possesses improved high frequency performance.
Another object of the invention is to provide an HBT construction that possesses a lower base to collector capacitance than previously thought possible.
A heterojunction bipolar transistor contains an emitter, base and collector; with each of the emitter, base and collector comprising a non-rectangular parallelogram in geometry. More specifically, the transistor is formed of a crystalline material which contains crystal planes [0 0 1], [0 0 {overscore (1)}], [0 1 0], [0 {overscore (1)} 0 ], [0 1 1] and [0 {overscore (1)} {overscore (1)}] and in which the foregoing non-rectangular parallelogram geometry of the emitter, base and collector contains one pair of parallel sides oriented parallel to said (0 0 1) crystal plane (or [0 0 {overscore (1)}] crystal plane) and the second pair of parallel sides oriented parallel to said [0 1 1] plane. As a consequence in one direction the side edges of the collector undercut the base and in the other direction the side edges of the collector slope down from the edge of the base and outwardly of that base edge. Insulated electrical leads extend along the sloped edge of the collector and are firmly supported. While the undercutting of the base reduces base to collector capacitance considerably.
The foregoing and additional objects and advantages of the invention together with the structure characteristic thereof, which was only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.